Using a Behringer 1204USB mixer with GarageBand on an iMac to record radio shows as live
I'm wanting to record my regular radio show using a Behringer 1204USB mixer and GarageBand. I want to record the show 'as live' rather than in blocks of audio on separate tracks. I have the mixer connected via USB and stereo splitter cable from the Line In stereo aux sends in the mixer and then connected to the iMac via a 3.5mm headphone jack. I have successfully been able to record my speech into GarageBand.
I'm wanting to play in music to the show using channel 7/8 of the mixer (the stereo channel) with music content coming from my iTunes. Problem I have is that when I play the recording back my speech comes out but the music I play is not audible on the track. I figure I've not clicked on the right setting in GarageBand or alternatively have a button on the mixer either pressed in or not pressed in.
I've watched endless YouTube tutorials to try and work this out so far to no avail. Any help/suggestions as to what might be the issue or issues here would be very welcome. I'm new to mixers and have used GarageBand before now for recording radio shows using a USB condensor mic and for editing only so I'm far from an expert on either the Behringer 1204 or GarageBand.
Many thanks.
iMac 21.5", macOS 10.15